See bug 100396. Eike added new parameters for compatibility with excel. We should create documentation.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/help/commit/?id=54980af34a59abbdabb15b22a7aaf4a808762fc8 tdf#100410 Update help page for WEEKDAY()